Algorithms, Abstraction, and Functions Abstraction "The - PowerPoint PPT Presentation

Algorithms, Abstraction, and Functions

Abstraction "The essence of abstractions is preserving information that is relevant in a given context, and forgetting information that is irrelevant in that context." John V. Guttag, Introduction to Computation and Programming Using Python

Abstraction "The essence of abstractions is preserving information that is relevant in a given context, and forgetting information that is irrelevant in that context." John V. Guttag, Introduction to Computation and Programming Using Python

• Think of a task or problem where: – you used to have to think about each step of the problem – but now it's automatic and you don't need to think about each step anymore.

Supercalifragilisticexpialidocious! Supercalifragilisticexpialidocious! Even though the sound of it Even though the sound of it Is something quite atrocious Is something quite atrocious If you say it loud enough If you say it loud enough You’ll always sound precocious You’ll always sound precocious Supercalifragilisticexpialidocious! Supercalifragilisticexpialidocious! Um diddle diddle diddle um diddle ay Um diddle diddle diddle um diddle ay Um diddle diddle diddle um diddle ay! Um diddle diddle diddle um diddle ay! Because I was afraid to speak He traveled all around the world And everywhere he went When I was just a lad He’d use his word and all would say My father gave me nose a tweak And told me I was bad “There goes a clever gent” When Dukes and maharajas But then one day I learned a word That saved me achin’ nose Pass the time of day with me I say me special word and then The biggest word I ever heard They ask me out for tea And this is how it goes: Oh! Supercalifragilisticexpialidocious! Even though the sound of it Is something quite atrocious If you say it loud enough You’ll always sound precocious Supercalifragilisticexpialidocious!

Supercalifragilisticexpialidocious! Supercalifragilisticexpialidocious! Even though the sound of it Even though the sound of it Is something quite atrocious Is something quite atrocious If you say it loud enough If you say it loud enough You’ll always sound precocious You’ll always sound precocious Supercalifragilisticexpialidocious! Supercalifragilisticexpialidocious! Um diddle diddle diddle um diddle ay Um diddle diddle diddle um diddle ay Um diddle diddle diddle um diddle ay! Um diddle diddle diddle um diddle ay! Because I was afraid to speak He traveled all around the world And everywhere he went When I was just a lad He’d use his word and all would say My father gave me nose a tweak And told me I was bad “There goes a clever gent” When Dukes and maharajas But then one day I learned a word That saved me achin’ nose Pass the time of day with me I say me special word and then The biggest word I ever heard They ask me out for tea And this is how it goes: Oh! Supercalifragilisticexpialidocious! Even though the sound of it Is something quite atrocious If you say it loud enough You’ll always sound precocious Supercalifragilisticexpialidocious!

Abstraction in Programming: Functions • A function is used to assign a name to a mini- program within a larger program. • Any time you want to run the mini-program, you can use the name of the function instead of repeating the code for the mini-program. • To use a function, we must define it first.

Defining a function Gives your function a name so it can be run later Syntax: def name_of_function (): statement # Notice how these lines are statement # indented. A group of indented statement # lines is called a block. ... # This is how Python knows which # lines of code go with the # function. Pick a name for your function that describes what it does! (Just like you pick variable names that describe what the variable holds.)

Defining a function Gives your function a name so it can be run later Syntax: def print_chorus (): print("Supercalifragilisticexpialidocious!") print("Even though the sound of it") print("Is something quite atrocious") print("If you say it loud enough") print("You'll always sound precocious") print("Supercalifragilisticexpialidocious!")

Functions • To use a function, we must define it first. • After defining a function, to run the code inside, you call the function.

Calling a function Runs the code inside the function definition Syntax: name_of_function () After defining a function, you can call it any number of times you want. Each time it is called Python acts as if you had typed in all of the lines of the function definition.

• You are in charge of dessert for Thanksgiving dinner. You decide to make two pumpkin pies and an apple pie. • Write a program that defines three functions: – make_apple() should print a description of how to make an apple pie. – make_pumpkin() should print a description of how to make a pumpkin pie. – cook_dessert() should call make_apple() and make_pumpkin() appropriately to make the pies.

The main() function • Python programs usually include a main() function that is the first function that runs when the program begins. – This function is in charge of calling any other functions. • This is not (technically) required in Python, but is a good habit. – Required in other languages like C++ and Java. – Required for CS 141! J

The main() function • From this point on, always define a main() function in your programs. • Always call the main() function as the last line of your program (your .py file).

def print_chorus(): print("Supercali…") (etc) def print_um_diddle(): print("Um diddle diddle…") (etc) def print_verse1(): print("Because I was afraid to speak…") (etc) # A function for the "main" program. def main(): print_chorus() # Print the chorus print_um_diddle() # Print the um diddles print_verse1() # Print the 1 st verse print_chorus() # Print the chorus again print_um_diddle() # Print the um diddles again print_verse2() # Print the 2 nd verse print_chorus() # Print the chorus the last time main() # Start the program

• Add code to the main() function so that the user is asked how many people are coming to dinner. – If 0-4 people are coming, you should make 1 pumpkin and 1 apple pie. – If 5-8 people are coming, you should make one extra pumpkin pie (2 pumpkin/1 apple). – If more than 8 people are coming, you should make another extra of each pie (4 total, 2 each). • Use if/elif/else to call make_pumpkin() and make_apple() the appropriate number of times. • Challenge : Write this code using three if statements rather than if/elif/else. Does this make the code easier or harder to understand?